Seventeen people were killed and multiple people were injured when a former student opened fire at Marjory Stoneman Douglas High School in Florida on Wednesday, prompting a massive police response and manhunt as frightened students hid in their classrooms.

CBS News has confirmed that the suspect has been identified as Nikolaus Cruz, 19. A law enforcement source briefed on the shooting said the suspect appears to have pulled the school fire alarm, causing chaos, and then started shooting, reports CBS News news senior investigative producer Pat Milton.